<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      padding: 0;
      margin: 0;
    }

    .box {
      width: 200px;
      height: 200px;
      background-color: #6ff;
      margin: 50px;
      overflow: hidden;
    }

    .sub {
      width: 100px;
      height: 100px;
      background-color: #00f;
      margin: 50px;
    }
  </style>
</head>

<body>
  <div class="box">
    <div class="sub"></div>
  </div>
</body>
<script>
  var box = document.getElementsByClassName("box")[0];
  var sub = document.getElementsByClassName("sub")[0];


  // fromElement 对于 mouseover 和 mouseout 事件，fromElement 引用移出鼠标的元素。 
  // toElement 对于 mouseover 和 mouseout 事件，该属性引用移入鼠标的元素。

  // e.fromElement 事件触发时,从哪个元素来:
  // e.toElement 事件触发时,到哪个元素去:

  sub.onmouseenter = function (e) {
    console.log("事件触发时,从哪个元素来:", e.fromElement);
    console.log("事件触发时,到哪个元素去:", e.toElement);
  }

  sub.onmouseleave = function (e) {
    console.log("事件触发时,从哪个元素来:", e.fromElement);
    console.log("事件触发时,到哪个元素去:", e.toElement);
  }




</script>

</html>